Interconnection system for transmitting power between electrical systems

ABSTRACT

An electrical interconnection system ( 100 ) comprises a variable frequency rotary transformer ( 102 ) and a control system ( 104 ). The control system ( 104 ) adjusts an angular position of the rotary transformer ( 102 ) so that measured power (P 1 ) transferred from a first electrical system ( 22 ) to a second electrical system ( 24 ) matches an inputted order power (P 0 ). The rotary transformer ( 102 ) comprises a rotor assembly ( 110 ) and a stator ( 112 ), with the control system ( 104 ) adjusting a time integral of rotor speed over time. The control system ( 104 ) includes a first control unit ( 107 ) and a second control unit ( 108 ). The first control unit ( 107 ) compares the input order power P 0  to the measured power P 1  to generate a requested angular velocity signal ω 0 . The second control unit ( 108 ) compares the requested angular velocity signal ω 0  to a measured angular velocity signal ω r  of the rotary transformer to generate a converter drive signal T 0  to a torque control unit ( 106 ), thereby controlling the angular positioning (θ r ) of the rotor assembly ( 110 ) relative to the stator ( 112 ). In various embodiments, the torque control unit ( 106 ) is integrated in the rotor assembly ( 110 ) and stator ( 112 ) of the rotary transformer ( 102 ).

This application is a continuation of U.S. patent application Ser. No.08/550,941 Oct. 31, 1995, now abandoned by Mark A. Runkle and entitled“INTERCONNECTION SYSTEM FOR TRANSMITTING POWER BETWEEN ELECTRICALSYSTEMS”, which in turn is a continuation-in-part of U.S. patentapplication Ser. No. 08/426, 201 filed Apr. 21, 1995 now abandoned byMark A. Runkle and entitled “INTERCONNECTION SYSTEM FOR ELECTRICALSYSTEMS HAVING DIFFERING ELECTRICAL CHARACTERISTIC” (abandoned), and isrelated to U.S. patent application Ser. No. 08/550,940 entitled“ASYNCHRONOUS CONVERSION METHOD AND APPARATUS FOR USE WITH VARIABLESPEED TURBINE HYDROELECTRIC GENERATION”, all of which are incorporatedherein by reference.

BACKGROUND

1. Field Of Invention

This invention pertains to control of electrical power transmission, andparticularly to transmission of power between electrical systems.

2. Related Art and Other Considerations

Some electrical transformers, for example tap-changing transformers suchas variacs, merely vary voltage. Other transformers, known as stationaryphase shifting transformers, can divert power and move power through atorque angle.

Mere voltage-varying transformers and stationary phase shiftingtransformers may be adequate for interconnecting two electrical systemsoperating at the same electrical frequency, or for transmission within autility company. However, such transformers are incapable of interfacingtwo electrical systems operating a differing frequency (e.g,inter-utility transfers of electricity).

There exist a number of areas in the world where interconnectionsbetween power systems require an asynchronous link. For some of theseareas the power systems have different nominal frequencies (e.g , 60 Hzand 50 Hz). Even for interconnections in other systems having the samenominal frequency, there is no practical means of establishing asynchronous link having enough strength to permit stable operation in aninterconnected mode.

The prevailing technology for accomplishing an asynchronousinterconnection between power systems is high voltage direct current(HVDC) conversion.

FIG. 8 is a one-line diagram schematically illustrating a prior art HVDCinterconnection system 820. FIG. 8 shows interconnection system 820connecting a first or supply system 822 (shown as AC Power System #1)and a second or receiver system 824 (shown as AC Power System #2). ACPower System #1 is connected to interconnection system 820 by lines 826for supplying, in the illustrated example, a three-phase input signal offrequency F1 (F1 being the frequency of supply system 822).Interconnection system 820 is connected by lines 828 to receiver system824, with lines 828 carrying a three-phase output signal of frequency F2from interconnection system 820 to receiver system 824.

HVDC interconnection system 820 of FIG. 8 includes a back-to-back DClink 830 situated between bus bars 832 and 834. Bus bar 832 is connectedto supply lines 826 and to reactive compensation bus 842. Bus bar 834 issimilarly connected to lines 828 and to reactive compensation bus 844.

Each side of back-to-back DC link 830 includes two transformers (e.g.,transformers YY and YΔ on the first system side; transformers YY and ΔYon the second system side) and a 12 pulse converter group. Asillustrated in FIG. 8, the 12 pulse converter group for the first sideof link 830 includes two six pulse converter groups 850 and 852; the 12pulse converter group for the second side of link 830 includes two sixpulse converter groups 860 and 862. As a three phase group isillustrated, each converter group includes six thyristors connected in amanner understood by the man skilled in the art. Smoothing filter 864 isconnected between converter groups 850 and 860.

Also shown in FIG. 8 are reactive power supply systems 870 and 880connected to reactive compensation buses 842 and 844, respectively.Reactive power supply system 870 includes a shunt reactor 871 connectedto bus 842 by switch 872, as well as a plurality of filter branches873A, 873B, 873C connected to bus 842 by switches 874A, 874B, and 874C,respectively. Similarly, reactive power supply system 880 includes ashunt reactor 881 connected to bus 844 by switch 882, as well as afilter branches 883A, 883B, 883C connected to bus 844 by switches 884A,884B, and 884C, respectively. Although three such filter branches873A-873C and 883A-883C have been illustrated, it should be understoodthat a greater number of filter branches may reside in each reactivepower supply system 870, 880.

For any given HVDC installation, reactive power supply systems such assystems 870 and 880 are difficult to design and are expensive. Moreover,there are a large number of switched elements that have to be carefullycoordinated with a given power level. Various constraints aresimultaneously imposed, such as keeping harmonic performance below arequisite level (i.e., harmonic performance index) and yet maintainingreactive power between limits, all the while essentially constantlyswitching the filters in systems 870 and 880 as power changes.Concerning such restraints, see (for example) Larsen and Miller,“Specification of AC Filters for HVDC Systems”, IEEE T&D Conference, NewOrleans, April 1989.

Thus, HVDC is complicated due e.g., to the need to closely coordinateharmonic filtering, controls, and reactive compensation. Moreover, HVDChas performance limits when the AC power system on either side has lowcapacity compared to the HVDC power rating. Further, HVDC undesirablyrequires significant space, due to the large number of high-voltageswitches and filter banks.

Prior art rotary converters utilize a two-step conversion, having both afully-rated generator and a fully-rated motor on the same shaft. Rotaryconverters have been utilized to convert power from AC to DC or from DCto AC. However, such rotary converters do not convert directly from ACto AC at differing frequencies. Moreover, rotary converters runcontinuously at one predetermined speed (at hundreds or thousands ofRPMS), acting as motors that actually run themselves. Prior art rotaryconverters accordingly cannot address the problem of interconnecting twoelectrical systems that are random walking in their differing frequencydistributions.

In a totally different field of technical endeavor, the literaturedescribes a differential “Selsyn”-type drive utilized for speed controlof motors. See Puchstein, Llody, and Conrad, Alternating-CurrentMachines, 3rd Edition, John Wiley & Sons, Inc., New York, pp. 425-428,particularly FIG. 275 on page 428, and Kron, Equivalent Circuits ofElectric Machinery, John Wiley & Sons, Inc., New York, pp. 150-163,particularly FIG. 9.5a on page 156. The literature cites thedifferential Selsyn drive only in the context of speed control ofmotors, i.e., motor speed control via relative speed adjustment betweena motor and generator. Moreover, the differential Selsyn drive has a lowbandwidth and makes no effort to dampen rotor oscillations.

SUMMARY

An electrical interconnection system comprises a rotary transformer anda control system. The control system adjusts an angular position of therotary transformer so that measured power transferred from a firstelectrical system to a second electrical system matches an inputtedorder power. The rotary transformer comprises a rotor assembly and astator, with the control system adjusting a time integral of rotor speedover time.

The control system includes a first control unit and a second controlunit. The first control unit compares the input order power to themeasured power to generate a requested angular velocity signal Thesecond control unit compares the requested angular velocity signal to ameasured angular velocity signal of the rotary transformer to generate aconverter drive signal, thereby controlling the angular positioning ofthe rotor assembly relative to the stator.

The rotary transformer comprises a rotor connected to the firstelectrical system and a stator connected to the second electricalsystem. A torque control unit or actuator rotates the rotor in responseto the drive signal generated by the control system.

The bandwidth of the control system is such to dampen oscillations(natural oscillations of the rotor including its reaction to thetransmission network into which it is integrated). The bandwidth of thefirst (slow) control unit is chosen to be below the lowest natural modeof oscillation; the bandwidth of the second (fast) control unit ischosen to be above the highest natural mode of oscillation. As usedherein, the bandwidth of a control unit or control system refers to thespeed of response of a closed-loop feedback unit or system.

The first and second electrical systems may have a differing electricalcharacteristic (e.g., frequency or phase). The controllerbi-directionally operates the rotary transformer at a variable speed fortransferring power from the first electrical system to the secondelectrical system or vise versa (i.e., from the second electrical systemto the first electrical system).

In some embodiments, the torque control unit (actuator) is a motor. Insuch embodiments, the torque control unit may either directly drive therotor, or interface with the rotor via a gear. In one particularembodiment, the gear is a worm gear.

In other embodiments, the torque control unit is integrated in the rotorassembly and stator of the rotary transformer. In such embodiments, thefunction of the torque control unit is accomplished by providing twosets of windings on both the rotor and the stator, a first set ofwindings on the rotor and stator having a different number of poles(e.g., 2 poles) than a second set of windings on the rotor and stator(e.g., 4 or more poles). The embodiments in which the torque controlunit is integrated in the rotor assembly and stator of the rotarytransformer include a squirrel cage inductor embodiment; DC-excitedrotor (synchronous) embodiment; and, a wound rotor AC embodiment.

The interconnection system of the present system is utilizable in asubstation for connecting asynchronous electrical systems, such as firstand second power grids having differing electrical frequencies. Theinterconnection system of the invention not only transfers power, butcan also modify power rapidly by accomplishing phase shift under load.

In the present invention, mechanical torque of the rotary transformer iscontrolled to achieve an ordered power transfer from stator to rotorwindings. The present invention contrasts with prior art techniqueswhich controlled power transfer from rotor to stator windings for thepurpose of controlling torque applied to the load (and thereby itsspeed). Moreover, in the present invention, both rotor and statorwindings are rated for full power transfer, whereas in prior artapplications the rotor winding was rated only for a small fraction ofthe stator winding.

Importantly, the present invention avoids the prior art HVDC need toclosely coordinate harmonic filtering, controls, and reactivecompensation. The present invention also advantageously provides aone-step conversion.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings in which reference characters refer to the same partsthroughout the various views. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention.

FIG. 1 is schematic view of an electrical power interconnection systemaccording to an embodiment of the invention.

FIG. 2 is a partial schematic, partial perspective view of an electricalpower interconnection system according to an embodiment of theinvention.

FIG. 3A is a side view of an electrical power interconnection systemaccording to an embodiment of the invention which utilizes a worm gear.

FIG. 3B is top view of the electrical power interconnect system of FIG.3A.

FIG. 4 is a schematic view of a substation for electricalinterconnecting a first electrical system and a second electricalsystem.

FIG. 5 is a graph showing torque-control requirements of the electricalpower interconnection system of the present invention.

FIG. 6 is a graph showing a capability curve of the electrical powerinterconnection system of the present invention.

FIG. 7A is a top schematic view of an embodiment wherein a torquecontrol unit is integrated in the rotor assembly and stator of therotary transformer

FIG. 7B is a top schematic view of an embodiment wherein a torquecontrol unit is integrated in the rotor assembly and stator of therotary transformer in a squirrel cage inductor configuration.

FIG. 7C is a top schematic view of an embodiment wherein a torquecontrol unit is integrated in the rotor assembly and stator of therotary transformer in a DC-excited rotor (synchronous) configuration.

FIG. 7D is a top schematic view of an embodiment wherein a torquecontrol unit is integrated in the rotor assembly and stator of therotary transformer in a wound rotor AC configuration.

FIG. 8 is a one-line diagram schematically illustrating a prior art HVDCinterconnection system.

FIG. 9 is a phasor diagram illustrating phasors of the interconnectionsystem of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrical power interconnection system 100 whichincludes a variable frequency transformer 102 and a control system 104.As described in more detail below with reference to FIG. 2, variablefrequency transformer 102 is connected by 3-phase lines RA, RB, RC(included in line 26) to first AC Power system 22 and by 3-phase linesSA, SB, and SC (included in line 28) to second AC Power System 24. Asalso explained below, the first electrical system and the secondelectrical system may have and likely do have a differing electricalcharacteristic, such as differing electrical frequency.

As shown in FIG. 1, variable frequency rotary transformer 102 includesboth a rotary transformer assembly 105 and a torque control unit 106(also known as the rotor drive section). Details of rotary transformerassembly 105 and torque control unit 106 are below described in moredetail in connection with FIG. 2.

As also shown in FIG. 1, control system 104 includes both a slow powercontrol unit 107; a fast speed control unit 108; and a power transducer109. Slow power control unit 107 is connected to receive the voltage V₁carried on line 26 from first AC Power System 22 and the voltage V₂carried on line 28 to second AC Power System 24, as well as (via powertransducer 109) a signal indicative of measured power P₁ transmitted online 26. Slow power control unit 107 is also connected to receive apower order input signal P₀ and to output a signal ω₀ to fast speedcontrol unit 108. Fast speed control unit 108 in turn receives a signalω_(r) from speed transducer 111 and outputs a drive signal T₀ to torquecontrol unit 106.

As shown in more detail in FIG. 2, rotary transformer assembly 105includes both a rotor subassembly 110 and a stator 112. Rotorsubassembly 110 includes collector rings 114 (also known as slip rings)and rotor cage section 116. Three-phase lines RA, RB, RC leading fromfirst AC Power System 22 are connected to collector rings 114;three-phase lines SA, SB, and SC leading to second AC Power System 24are connected to stator 112. Rotor subassembly 110 has speed transducer111 mounted proximate thereto for generating the angular velocity signalω_(r) indicative of the angular velocity of the rotor.

As shown in FIG. 2 and understood by the man skilled in the art, in theillustrated embodiment rotary transformer assembly 105 is wound withsixty degree phase belts, with rotor windings being labeled as RA+, RC−,RB+, RA−, RC+, and RB− and stator windings labeled as SA+, SC−, SB+,SA−, SC+, and SB−. It should be understood that the invention is notlimited to a sixty degree phase belt-wound system, rather the principlesof the invention are applicable for rotary transformer assemblies ofphase two and greater.

Rotor assembly 110 is rotatable about its axis RX in both clockwisedirection CW and counter-clockwise direction CCW. Rotation of rotorassembly 110 is effected by rotor drive section 106.

Rotor drive section 106 is shown symbolically in FIG. 2 as a cylindricalsection mounted on rotor assembly 110. Thus, rotor drive section 106 ofFIG. 2 generally depicts various alternative and different types ofdrive mechanisms for causing rotation of rotor assembly 110. In someembodiments, rotor drive section 106 includes an actuator and some typeof linkage (e.g., gearing) which interfaces with rotor assembly 110. Forexample, in one embodiment rotor drive section 106 comprises a worm geardrive arrangement as shown in FIG. 3A and FIG. 3B and discussedhereinafter. In other embodiments, rotor drive section 106 comprises anactuator such as a stepper motor acting through a radial (e.g, spur)gear, a direct drive arrangement, a hydraulic actuator turning a gear onrotor assembly 110, or a pneumatic actuator turning a gear on rotorassembly 110. In yet other embodiments generally illustrated in FIG. 7,the function of the torque control unit (shown as element 106″) isaccomplished by providing two sets of windings on both the rotor and thestator, a first set of windings on the rotor and stator having adifferent number of poles (e.g., 2 poles) than a second set of windingson the rotor and stator (e.g., 4 or more poles). Thus, any suitabledrive mechanism may be employed for rotor drive section 106 so long asit is compatible with the closed loop angular position of rotor assembly110 as described herein.

Control system 104 bi-directionally operates the rotor assembly 110(through rotor drive section 106) for transferring power from first ACPower System 22 to second electrical system 24, or vise-versa. Inoperation, an operator sets the power order input signal P₀ inaccordance with a predetermined power transfer requirement prearrangedbetween AC Power Systems 22 and 24. Setting power order input signal P₀can be accomplished by adjusting a knob or inputting data at an operatorcontrol panel or operator workstation CP to generate signal P₀indicative of the ordered power. In the particular embodimentillustrated in FIG. 1, control panel CP is remotely located frominterconnection system 100.

Slow power control unit 107 compares the power order input signal P₀with the measured power transfer signal P₁ in order to produce arequested angular velocity signal ω₀. The measured power transfer signalP₁ is obtained from the three lines 26 by power transducer 109. Powertransducer 109 can be any one of a number of conventional instruments,with the man skilled in the art readily appreciating how to obtainsignal P₁.

In the illustrated embodiment, slow power control unit 107 is anintegrator which measures a difference between order power signal P₀ andmeasured power signal P₁ (i.e., P₀-P₁) and puts the result into anintegrator function to generate requested angular velocity signal ω₀.Slow power controller 107 has a very low gain to prevent interference ofnatural AC system dynamics generally occurring above 3 rad/sec (asdescribed below).

Fast speed controller 108 receives both the requested angular velocitysignal ω₀ and a measured angular velocity signal ω_(r). The requestedangular velocity signal ω₀ is generated by slow speed controller 107 asaforedescribed. The measured angular velocity signal ω_(r) is obtainedfrom speed transducer 111. Fast speed controller 108 generates a drivesignal (also known as the torque order signal T₀) on line 134 so thatω_(r) promptly equals ω₀. The man skilled in the art knows how tooperate conventional motor drivers as fast speed controller 108 to usesignals ω_(r) and ω₀ to generate the drive signal T₀.

Thus, fast speed controller 108 operates to adjust the drive signal T₀on line 134 to torque control unit 106 such that the actual speed ω_(r)of rotor assembly 110 follows the ordered speed ω₀. The closed-loopbandwidth of fast speed controller 108 should exceed the highest naturaloscillatory frequency of rotor assembly 110, including its reaction tothe transmission network into which it is integrated, and is generallyless than 100 rad/sec. Typically, the natural modes of oscillation willrange from about 3 rad/sec through 50 rad/sec, and are usually less than30 rad/sec. In connection with the bandwidth (speed of response) of fastspeed controller 108, in the illustrated embodiment, a phase lag from achange in ordered speed ω₀ to actual speed ω_(r) of rotor assembly 110is less than 90 degrees for sinusoidal disturbances. Ensuring thisbandwidth of response will in turn ensure that all such natural modes ofoscillation will experience beneficial damping from the control system.

The directionality (e.g, polarity) of the drive signal T₀ on line 134 isin accordance with the direction of power flow (e.g., in accordance withwhether power is flowing from AC Power System 22 to AC Power System 24or vise-versa). The magnitude of the drive signal T₀ on line 134 is usedby rotor drive section 106 to increase or decrease the speed of rotorassembly 110 in order to match the difference between the frequencies ofAC Power System 22 and AC Power System 24.

As shown in FIG. 2, drive signal T₀ on line 134 is applied to torquecontrol amplifier 150. Power is supplied to torque control amplifier 150by torque control power source 152, whereby using drive signal T₀ online 134 the torque control amplifier 150 outputs the three phasesignals TA, TB, and TC to torque control unit 106. As used herein and inthis art, TA refers collectively to TA+and TA−, TB refers collectivelyto TB+ and TB−, and so forth.

FIG. 9 is a phasor diagram drawn with respect to a reference phasorV_(ref). FIG. 9 shows voltage phasor V₁ representing the voltage V₁ atAC Power System 22, voltage phasor V₂ representing voltage V₂ at ACPower System 24, as well as the phase angle θ₁ of ac voltage on lines 26with reference to phasor V_(ref), the phase angle θ₂ of ac voltage onlines 28 with reference to phasor V_(ref), and θ_(r). The angularpositioning θ_(r) of the rotor assembly 110 relative to the stator 112is also shown in FIG. 2, it being understood from conventional practicethat θ_(r) is zero when RA+lines up exactly with SA+.

An objective of interconnection system 100 is to cause the rotationalspeed and angular position θ_(r) of variable frequency transformer 102to be such that a desired power (i.e., indicated by order power signalP₀) is transferred through interconnect system 100 between AC PowerSystem 22 and AC Power System 24. In essence, interconnect system 100controls angle θ_(r) (see FIG. 9) so that measured power signal P₁matches the order power signal P₀. Drive signal T₀ on line 134 is usedto adjust the angular relationship θ_(r) of rotor assembly 110 relativeto stator 112, so that the speed and angle of rotary transformer 102enable transmission of power at the ordered power level.

The power transfer through interconnect system 100 is approximated byEquation 1:

P ₁ =V ₁ V ₂sin(θ₁−θ₂+θ_(r))/X ₁₂  Equation 1

wherein

P₁=power through interconnect system 100;

V₁=voltage magnitude on lines 26

V₂ =voltage magnitude on lines 28

θ₁=phase angle of ac voltage on lines 26 with reference to referencephasor V_(ref)

θ₂=phase angle of ac voltage on lines 28 with reference to referencephasor V_(ref)

θ_(r)=phase angle of rotor assembly 110 with respect to stator

X₁₂=total reactance between lines 26 and 28.

There is a maximum theoretical power transfer possible throughinterconnect system 100 in either direction. The absolute magnitude ofthe theoretical power transfer is provided by Equation 2:

P _(max) =V ₁ V ₂ /X ₁₂  Equation 2

which occurs when the net angle is near 90° in either direction, asunderstood from Equation 3:

θ_(net)=θ₁−θ₂+θ_(r)=±90°  Equation 3

For stable operation, angle θ_(net) must have an absolute valuesignificantly less than 90°, which means that power transfer will belimited to some fraction of the maximum theoretical level given byEquation 2. Within this range, the power transfer follows a monotonicand nearly linear relationship to the net angle, which can beapproximated by Equation 4:

P ₁ ≅P_(max)θ_(net)  Equation 4

The angles of the ac voltage phasors of FIG. 9 are given by the timeintegrals of their respective frequencies, while the angle of rotorassembly 110 is given by the integral of it speed over time, asdemonstrated by Equation 5:

P ₁≅P_(max)[∫(ω₁(t)−ω₂ (t)) dt+∫(ω_(r)(t)) dt]  Equation 5

where

ω₁(t)=frequency of ac voltage on line 26 as a function of time;

ω₂(t)=frequency of ac voltage on line 28 as a function of time;

ω_(r)(t)=frequency of rotor assembly 110 as a function of time.

Thus, the through power is directly affected by the time-integral ofshaft speed of rotor assembly 110 over time. This characteristic permitsa power-regulating control loop to be implemented in the presentinvention by measuring the through power (P₁) and adjusting the orderfor shaft speed (ω₀). By keeping the bandwidth of this power regulatorwell below that of the lowest oscillatory mode on the system (typicallybelow 3 rad/sec), the objective of damping rotor oscillations will notbe compromised.

The fast power-limit function is used to override the normal slow powerregulator when the measured power exceeds the limit computed frommeasured voltages. The limit will be some fraction of the maximumtheoretical power as indicated by Equation 6:

P _(LIMIT) =F _(LIMIT) P _(max)  Equation 6

wherein

P_(LIMIT)=the power limit (applied in either direction);

F_(LIMIT)=the maximum allowed fraction of theoretical power.

As used herein, phase refers to electrical phase. If there are more thantwo poles, the relationship between mechanical degrees on rotor assembly110 and electrical degrees is

mechanical degrees=2/#poles*electrical degrees.

Phase shift is accomplished by physically displacing rotor assembly 110relative to stator 120. The angular position of rotor assembly 110 maybe maintained, advanced, or retarded at will. The phase shift isaccomplished by changing rotor angular position and thus modifying themutual inductances among the phases of interconnection system 100.

The number of poles (NP) rotary transformer 105 is typically dependentupon system parameters, such as the number of possible air gaps.However, the number of poles (NP) of the system influences the number ofmechanical degrees (NMD) of rotor angular displacement necessary totransfer power for a given electrical frequency differential (EFD), asindicated by the expression NMD=2*EFD/NP. Thus, a high pole number (highNP) can greatly reduce the number of mechanical degrees (NMD) of angularshift required to achieve the electrical shift. For example, a −30 to+30 degree electrical shift is only −2 to +2 degrees of mechanical shifton a 30 pole wound-rotor motor. By reducing the mechanical angle to beshifted, the forces required can be greatly reduced, or conversely theresponse time greatly increased, to achieve the desired shift.

FIG. 3A and FIG. 3B show electrical power interconnection system 100having a specific rotor drive section (torque control unit) 106′. Rotordrive section 106′ employs worm gear 160 meshing with rotor radial gear162 as a linkage, and additionally employs worm gear servo driver 164(e.g., a stepper motor) as an actuator. In addition, FIG. 3A and FIG. 3Bshow specific mounting structure of rotor assembly 110, particularlythrust and radial bearing 170 and top radial bearing 172 whichfacilitate both placement and rotation of rotor assembly 110. Anadvantage of rotor drive section 106′ is that the worm gear drive tendsto be self-locking. Should its associated servo driver 164 fail to turn,rotor assembly 110 will be locked in position until the electrical phaseerror reaches 360 degrees. At such time protective relaying will takeinterconnection system 110 off-line.

As mentioned above, in other embodiments other types of mechanisms areutilized for rotor drive section 106. Whereas rotor drive section 106′of FIG. 3A and FIG. 3B provides a solid connection of rotor assembly 110to worm gear 160, such solid connection need not necessarily occur forother embodiments. For example, in one embodiment a torsionspring/damper system is inserted between the worm gear 160 and rotorassembly 110 to adjust mechanical dynamics. In such a system, the wormgear-based rotor drive section 106′ displaces the phase (for example, 20electrical degrees), then the combination of electrical energy andmechanical energy is tuned to match the time constants of the load beingfed. The result is a uniform power input on the utility side andstabilization of the system load.

FIG. 7B, FIG. 7C, and FIG. 7D show specific implementations of rotordrive sections, generally represented by FIG. 7A, which implementationsare illustrated as rotor drive sections 106″A, 106″B, and 106″C,respectively. FIG. 7B, FIG. 7C, and FIG. 7D are illustrations ofembodiments in which the rotor drive sections are integrated in therotor assembly 110 kid stator 112 of rotary transformer 110. Inparticular, FIG. 7B illustrates a squirrel cage inductor embodiment;FIG. 7C illustrates DC-excited rotor (synchronous) embodiment; and, FIG.7D illustrates a wound rotor AC embodiment.

Referring collectively now to the embodiments of FIG. 7B, FIG. 7C, andFIG. 7D, the rotor drive sections 106″A, 106″B, and 106″C, respectively,all employ a two-pole rotor/stator configuration within a four-polerotor/stator configuration. In these embodiments, rotor assembly 110″ isseen to have, on its outer periphery, the same two-pole rotor windingsas shown in FIG. 2 (such common rotor windings being shaded). Inaddition, rotor assembly 110″ has eight four-pole rotor windings(located at a smaller radius from the center of rotor assembly 110″ thanthe two-pole rotor windings, the four-pole rotor windings beingunshaded). Stator 112″ has, on its inner periphery, the same two-polestator windings as shown in FIG. 2 (such common stator windings alsobeing shaded). In addition, stator 112″ has eight four-pole statorwindings (located at a larger radius from the center of rotor assembly110″ than the two-pole stator windings, the four-pole stator windingsbeing unshaded). In the embodiments of FIG. 7B, FIG. 7C, and FIG. 7D,the four-pole stator windings are connected to the lines TA+, TB+, TC+,TA−, TB−, TC− emanating from torque control amplifier 150 (see FIG. 2),and the connections of RA, RB, RC, SA, SB, SC are as shown in FIG. 2.

In the squirrel cage embodiment of FIG. 7A, the four-pole rotor windingsare shorted upon themselves to form a squirrel cage induction motor.

In the DC-excited rotor (synchronous) embodiment of FIG. 7C, speedcontrol unit 108 generates a further signal E_(fdo) (field voltage)which is applied to exciter amplifier 700B. Exciter amplifier 7001Bderives its power from exciter power source 702B, and outputs signalsDC+ and DC− to a slip ring assembly 114B having two slip rings.Placement of the slip rings in slip ring assembly 114B is understoodfrom collector rings 114 of FIG. 2.

In the wound rotor AC embodiment of FIG. 7D, speed control unit 108generates a further signal TR₀ (rotor current signal) which is appliedto rotor excitation amplifier 700C. Rotor excitation amplifier 700Cgenerates the three phase signals TRA, TRB, and TRC which are applied tothree slip rings comprising slip ring assembly 114C.

In the embodiments of FIG. 7B, FIG. 7C, and FIG. 7D, the flux in the airgap (between rotor and stator) for the four-pole configuration rotateonly half as fast as the flux for the two-pole configuration. As aresult, the influence of the four-pole flux upon the two-poleconfiguration has no average value, but only a “beat” value. In otherwords, if the flux of the two-pole configuration were rotating at 1 Hz,and the flux of the four-pole configuration were rotating at 0.5 Hz, thetwo-pole configuration will see a beat frequency of 0.5 Hz.

Although the embodiments of FIG. 7A, FIG. 7B, and FIG. 7C show atwo-pole and four-pole configuration, it should be understood that thesecond configuration could be larger than four poles (e.g, increase thedifference of the number of poles of the two configurations), so as toincrease the beat frequency between the two configurations to keeppulsations low.

With the embodiments of FIG. 7B, FIG. 7C, and FIG. 7D average torque iscontrolled using both sets of windings (the two-pole windings and thefour-pole windings) independently. For example, as illustrated by theconnections in FIG. 7, the four-pole windings could be used to performthe same function of torque control unit 106 (also known as the rotordrive section) of FIG. 2.

Although advantageously avoiding slip rings, the squirrel cageembodiment of FIG. 7A sees average torque from both the “S” windings andfrom the “T” windings on stator 112. Consequentially, control unit 108must unscramble these two effects, which the man skilled in the art willreadily appreciate and how to resolve.

FIG. 4 shows incorporation of electrical power interconnection system100 of the present invention in a substation 200. Substation whichelectrical interconnects a first electrical system 222 and to a secondelectrical system 224. It should be understood that first electricalsystem 222 (labeled as AC Power System #1) and second electrical system224 (labeled as AC Power System #2) have differing electricalcharacteristic(s). In the illustration of-FIG. 4, both systems/utilities222 and 224 happen to operate at 230 kV. It should be understood thatother appropriate voltages are employed in other embodiments.

Power supplied by system 222 enters substation 200 of FIG. 4 throughseries power capacitor 230 (20 Mvar) and is stepped down via 100 MVAgenerator step-up (GSU) transformer 232 from 230 kV to 15 kV.Stepped-down power from transformer 232 is applied on input line 234 tovariable frequency transformer 102 of interconnection system 100. Asshown in FIG. 2, input line 234 is actually the three phase lines RA,RB, and RC connected to collector rings 114. An electrical field andpower are established on rotor assembly 110, are transferred to stator112, and transferred from stator 112 on output line 236 at 15 kV. Asunderstood from FIG. 2, output line 236 is actually the three phaselines SA, SB, and SC. Power output from stator 112 on output line 236 isstepped up at 100 MVA generator step-up (GSU) transformer 238 from 15 kVto 230 kV. Stepped-up power from transformer 238 is then applied throughseries power capacitor 240 (20 Mvar) to system 224.

As understood from the foregoing description of controller 104 inconnection with FIG. 2, as included in substation 200 the control system104 monitors the frequencies of both system 224 and system 222 as thosefrequencies wander through their differing and respective frequencyranges. As the power flow is monitored, controller 104 generates a drivesignal for adjusting the angular position of rotor assembly 110 so thatelectrical power may be transmitted from system 222 to system 224.

In the above regard, if system 222 were at 59.9 Hz and system 224 wereat 60.1 Hz, interconnection system 100 would require a 0.2 Hz change forpower transfer from system 222 to system 224. For a 2-pole device, therequisite rotational velocity for rotary transformer 105 would be120*(0.2)/1=12 revolutions per minute. Given the fact that thesefrequencies also fluctuate or wander, rotary transformer 105 shouldtypically also be capable of ±0.50 Hz, or a speed range from +30 to −30revolutions per minute (RPMs) for the 2-pole equivalent.

FIG. 5 shows the linear relationship between control torque and throughpower between the first and second electrical systems. For constantpower into the machine and constant load power factors, theelectromagnetic torque developed is constant. As the slip between thesystems increases, the RPM (ω_(r)) required to match them increases, andthe product of the torque and speed is power required by the drive.

FIG. 6 shows the practical capability curves of the drive system of thepresent invention. The relationship between control torque and throughpower was shown in FIG. 5. “Through Power” is a machine thermal ratingif maximum torque is provided by an electromechanical drive (e.g., amaximum thrust rating for worm gear bearing). “Through Power” isessentially material-limited, whether insulation class for a winding ora material stress allowable, respectively.

In the present invention, mechanical torque of the rotary transformer iscontrolled to achieve an ordered power transfer from stator to rotorwindings. The present invention contrasts with prior art techniqueswhich controlled power transfer from rotor to stator windings for thepurpose of controlling torque applied to the load (and thereby itsspeed). Moreover, in the present invention, both rotor and statorwindings are rated for full power transfer, whereas in prior artapplications the rotor winding was rated only for a small fraction ofthe stator winding.

Importantly, the present invention avoids the prior art HVDC need toclosely coordinate harmonic filtering, controls, and reactivecompensation. The present invention also advantageously provides aone-step conversion.

Advantageously, interconnection system 100 of the present inventionperforms continuous phase shifting by controlling rotor angle θ_(r)(i.e., the angular position of rotor assembly 110). Interconnectionsystem 100 permits continuous adjustment of electrical phase by virtueof its potential 360 degree rotation, making the system a very lowfrequency synchronous converter. Moreover, interconnection system 100can be repetitively displaced through large angular displacements toaccomplish very large electrical phase shifts in a rapidly changingpower condition on a large system.

Thus, unlike conventional rotary converters, rotor assembly 110 is notrotated at a constant angular velocity, but instead is rotated at acontinuously variable angular velocity as required by control system104. Moreover, bi-directional angular velocity is achieved as rotorassembly 110 is rotatable in both clockwise direction CW andcounter-clockwise direction CCW as shown in FIG. 2.

Whereas typical synchronous converters run at constant, uni-directionalangular velocity of hundreds or thousands of RPMs, rotary transformer105 of interconnection system 100 typically runs, forward or backward,at less than 50 RPM.

Interconnection system 100 provides accurate and reliable phase shiftcontrol, with the ability to follow frequency drift and control phase inall four quadrants of control. Thus, interconnection system 100 not onlytransfers power, but can also modify power rapidly by accomplishingphase shift under load.

Although described above for its interconnect function, interconnectionsystem 100 can also serve as an energy storage system. Interconnectionsystem 100 can be used to store energy by rotational inertia, to averagelarge pulsating loads, similar to those present in arc melters for steelprocessing.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various alterations in form and detail maybe made therein without departing from the spirit and scope of theinvention. For example, whereas in the foregoing description a supplyelectrical system has been illustrated as being connected to collectorrings 114 and a receiver electrical system has been illustrated as beingconnected to stator 112, it should be understood that these illustratedconnections can be reversed.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An electricalinterconnection system comprising: a rotary transformer for coupling toa first electrical system and to a second electrical system, the rotarytransformer comprising: a rotor connected to the first electricalsystem; a stator connected to the second electrical system; a controllerwhich adjusts an angular position of the rotary transformer, thecontroller comprising: a first control unit which compares an inputorder power signal P₀ to a measured power signal P₁ being transferredbetween the first electrical system and the second electrical system togenerate a requested angular velocity signal ω₀; a second control unitwhich compares the requested angular velocity signal ω₀ to a measuredangular velocity signal ω_(r) of the rotary transformer to generate adrive signal T₀.
 2. The system of claim 1, wherein the controlleradjusts an angular position of the rotary transformer so that apredetermined power is transferred from the first electrical system tothe second electrical system.
 3. The system of claim 1, wherein thecontroller has a bandwidth chosen to dampen inherent oscillations in theinterconnection system.
 4. The system of claim 1, wherein the firstelectrical system and the second electrical system are a respectivefirst electrical utility company and a second electric utility company.5. The system of claim 1, wherein the interconnection system furthercomprises a torque control unit for rotating the rotor.
 6. The system ofclaim 5, wherein the controller controls the torque control unit wherebythe rotor is rotated at a variable speed.
 7. The system of claim 6,wherein the controller controls the torque control unit whereby therotor is bi-directionally rotated at a variable speed.
 8. A substationfor electrically interconnecting a first electrical system and to asecond electrical system, the first electrical system and the secondelectrical system having a differing electrical characteristic, thesubstation comprising: a step-down transformer coupled to the firstelectrical system; a step-up transformer coupled to the secondelectrical system; a rotary transformer coupled to the step-downtransformer and to the step-up transformer, the rotary transformercomprising: a rotor connected to a first of the step-down and step-uptransformers; a stator connected to a second of the step-down andstep-up transformers; a controller which adjusts an angular position ofthe rotary transformer so that a predetermined power is transferred fromthe first electrical system to the second electrical system, thecontroller comprising: a first control unit which compares an inputorder power signal P₀ to a measured power signal P₁ being transferredbetween the first electrical system and the second electrical system togenerate a requested angular velocity signal ω₀; a second control unitwhich compares the requested angular velocity signal ω₀ to a measuredangular velocity signal ω_(r) of the rotary transformer to generate adrive signal T₀.
 9. The system of claim 8, wherein the controllerbi-directionally operates the rotary transformer at a variable speed fortransferring power from the first electrical system to the secondelectrical system.
 10. The system of claim 8, wherein the rotarytransformer comprises: a rotor connected to a first of the step-down andstep-up transformers; a stator connected to a second of the step-downand step-up transformers; and wherein the interconnection system furthercomprises an torque control unit for rotating the rotor.
 11. The systemof claim 10, wherein the controller controls the torque control unitwhereby the rotor is rotated at a variable speed.
 12. The system ofclaim 8, wherein the controller has a bandwidth chosen to dampeninherent oscillations in the interconnection system.
 13. An electricalinterconnection system comprising: a rotary transformer for coupling toa first electrical system and to a second electrical system, the rotarytransformer comprising: a rotor connected to the first electricalsystem; a stator connected to the second electrical system; a closedloop angular positioning control system which operates the rotarytransformer for transferring power from the first electrical system tothe second electrical system in response to a comparison between aninput order power signal P₀ and a measured power signal P₁ beingtransferred between the first electrical system and the secondelectrical system.
 14. A method of interconnecting two electricalsystems, the method comprising: coupling a rotor of a rotary transformerto a first electrical system and a stator of the rotary transformer to asecond electrical system; comparing an input order power signal P₀ and ameasured power signal P₁ being transferred between the first electricalsystem and the second electrical system to generate a requested angularvelocity signal ω₀; and adjusting an angular position of the rotarytransformer so that a predetermined power is transferred from the firstelectrical system to the second electrical system, the adjusting beingperformed by a closed loop angular positioning control system whichoperates the rotary transformer for transferring power from the firstelectrical system to the second electrical system.
 15. The method ofclaim 14, further comprising: comparing the requested angular velocitysignal ω₀ to a measured angular velocity signal ω_(r) of the rotarytransformer to generate a drive signal T₀.